net: stmmac: dwmac-rk: fix error handling in rk_gmac_powerup()

If phy_power_on() fails in rk_gmac_powerup(), clocks are left enabled.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>